In the catalytic reforming of naphthas to produce high-octane gasoline, and in the dehydrocyclization of C.sub.6 -C.sub.12 paraffinic and olefinic hydrocarbons to produce benzene and alkyl benzenes, catalysts comprising platinum supported on a chlorided alumina base have long been used. The chloride component contributes an acid function which promotes hydrocracking, paraffin isomerization and dehydrocyclization reactions. For high severity operations in which maximum dehydrocyclization of paraffins is desired, the concomitant hydrocracking reactions can severely reduce liquid yields and lead to excessive dry gas (C.sub.1 -C.sub.3) production. The isomerization of paraffins to more highly branched isoparaffins can also reduce yields of desired aromatic hydrocarbons, since paraffins which do not contain at least 6 carbon atoms with a straight chain of at least 5 carbon atoms will not readily cyclize, but tend instead to hydrocrack. For these and other reasons it would be desirable to moderate the acid function of the chloride component so as to reduce its hydrocracking and isomerization activity while retaining substantial dehydrocyclization activity. Even in cases where dehydrocyclization is not the main objective but in which some selective hydrocracking of heavy ends is desired, as for example in the reforming of high end-point naphthenic naphthas, it is often desirable to moderate the hydrocracking activity of the catalyst in order to reduce dry gas make.
In dehydrocyclization processes as such, it has been suggested at one extreme that instead of chloriding platinum-alumina catalysts, they should actually be essentially completely poisoned for cracking activity by the addition thereto of an alkali metal. While this practice does reduce cracking, it also drastically reduces dehydrocyclization activity, to the point that at the present time chlorided catalysts are found under proper conditions to give superior product distributions. The successful moderation of cracking activity, while retaining good dehydrocyclization activity of chlorided Pt--Al.sub.2 O catalysts is the primary objective of the present invention - an objective which we have found is not achieved by merely reducing the chloride content of the catalyst.
We have now discovered that prior art catalysts comprising chlorided platinum-alumina composites are substantially improved in dehydrocyclization selectivity as opposed to non-selective cracking by incorporating therein certain small amounts of cesium, less than the atomic proportion of chloride in the catalyst. The reason for this improvement is uncertain, but it can be hypothesized that cesium, being the most strongly alkaline of the metals, may selectively poison the strongest acid sites on the catalyst which promote non-selective cracking to dry gas. Its large atomic radius may also be an operative steric factor. Other alkali metals such as sodium are found to give some smaller degree of improved selectivity, but only at high temperatures entailing low liquid yields. Cesium however is effective over a wide range of temperatures and conversion levels.
In some naphtha reforming operations, it is economically desirable to maximize the yield of C.sub.6 -C.sub.8 aromatic hydrocarbons for the petrochemicals market. Maximizing aromatics production, particularly benzene, requires severe reforming conditions in order to achieve substantial dehydrocyclization of C.sub.6 -C.sub.12 paraffins, which conditions also tend to reduce C.sub.5.sup.+ yields as a result of cracking. The catalysts of this invention are particularly designed to achieve the objectives of high C.sub.6 -C.sub.8 aromatic yields and high C.sub.5.sup.+ yields in this type of reforming. They are also very active and selective for dehydrocyclization of the C.sub.6 -C.sub.12 paraffins and olefins having a straight chain of at least 5 carbon atoms, to produce products such as benzene, toluene, xylenes, ethylbenzene, etc. For these and other operations the catalyst also preferably contains a very small proportion of one or more stabilizing metals such as iridium, rhodium, rhenium, etc.